Power amplifiers may be integrated onto the same die as receiving or other types of amplifiers in many communications standards, such as wireless LAN. A switch permits transmission of signals through the antenna to/from either the power amplifier circuit or the receiving or other amplifier, depending on the needs of the communications circuit. When the power amplifier is transmitting signals, power is supplied to the power amplifier. When the power amplifier is not actively transmitting signals, power is not supplied to the power amplifier. Upon powering on, the power amplifier experiences a period of time during which the signal that is transmitted is changing and the transmission characteristics of the power amplifier are also changing, which is difficult to train or characterize by other circuit components or other circuits coupled to the power amplifier circuit. In some circumstances such as Wireless LAN, the receiving circuit that receives the signal transmitted from the power amplifier is arranged to determine the characteristics of the incoming signal from the power amplifier within a finite period of time after which the receiving circuit assumes that the signal changes remain consistent upon further receipt of the signal over time.
The time period during which the receiving circuit determines the characteristics of the incoming signal from the power amplifier may be relatively short, approximately 4 μs in some wireless LAN examples, compared to the time period during which the power amplifier is powering on and during which the transmission characteristics of the power amplifier signal are changing and the manner in which they are changing is also changing. This assumption of no further changes to the manner in which the signal is changing leads to the receiving circuit assuming that the power amplifier signal is not continuing to change in the same manner when in fact the characteristics of the power amplifier signal are continuing to change, which can lead to inaccuracies in how the signal is being processed by the receiving circuit.
To accommodate for changing transmission characteristics during the powering-on time period of the power amplifier, a reference circuit has been coupled to the power amplifier to generate a complementary reference current to the current flowing through the power amplifier. The complementary reference current produced by the reference circuit has an input of a voltage from a control source that is then converted to the reference current by the reference circuit. A reference circuit resistor sets both the steady state current value and the time constant or delay. A reference circuit capacitor value becomes fixed as a result of the fixed value for the reference circuit resistor. The steady state current value requires that the capacitor value be relatively large and that value can be upwards of 100 nF in some examples. It is not practical for such a large capacitor to be physically present on die and therefore it must be located off-die.
Many WLAN devices operate at high transmit power levels, typically between 16 and 20 dBm, and use a supply voltage which is typically 3.6V, to transmit signals between the device and a remote access point. High transmit power is required since the remote access point may be a considerable distance away from the WLAN device. Recently, devices incorporating features such as WiFi Direct® and personal hotspots have been implemented. In this operating mode, WLAN devices are often quite close to one another, and lower transmit power can be tolerated.
However, current WLAN devices do not have an option to efficiently operate at low power levels, which results in power being wasted during the closer proximity signal transmissions because the power amplifiers are configured to operate at the high power level. Such high power level operation leads to inefficient use of the power sources. Further, current WLAN devices that have been optimized for high power level operation are not capable of accurately correcting for start-up transients when a WLAN device power amplifier is operated at low power levels, which leads to inaccuracies when the signal is processed at the receiving circuit.
Therefore, advancements in the art that provide for WLAN devices with increased power efficiency that are size, cost, and power efficient are desirable.